> ## Documentation Index
> Fetch the complete documentation index at: https://tally.wharflab.com/llms.txt
> Use this file to discover all available pages before exploring further.

# powershell/PSUseConsistentParameterSetName

> Use consistent parameter set names and proper parameter set configuration.

`powershell/PSUseConsistentParameterSetName` is a PSScriptAnalyzer diagnostic emitted by tally for PowerShell snippets embedded in Dockerfiles.

| Property | Value            |
| -------- | ---------------- |
| Severity | Warning          |
| Category | PSScriptAnalyzer |
| Auto-fix | No               |

## Description

Parameter set names in PowerShell are case-sensitive, unlike most other PowerShell elements. This
rule ensures consistent casing and proper configuration of parameter sets to avoid runtime errors
and improve code clarity.

The rule performs five different checks:

1. **Missing DefaultParameterSetName** - Warns when parameter sets are used but no default is
   specified
2. **Multiple parameter declarations** - Detects when a parameter is declared multiple times in the
   same parameter set. This is ultimately a runtime exception - this check helps catch it sooner.
3. **Case mismatch between DefaultParameterSetName and ParameterSetName** - Ensures consistent
   casing
4. **Case mismatch between different ParameterSetName values** - Ensures all references to the same

   parameter set use identical casing
5. **Parameter set names containing newlines** - Warns against using newline characters in parameter
   set names

<Note>
  This rule isn't enabled by default. The user needs to enable it through settings.
</Note>

## How

* Use a `DefaultParameterSetName` when defining multiple parameter sets
* Ensure consistent casing between `DefaultParameterSetName` and `ParameterSetName` values
* Use identical casing for all references to the same parameter set name
* Avoid declaring the same parameter multiple times in a single parameter set
* Do not use newline characters in parameter set names

## Examples

### Problematic code

```powershell theme={null}
# Missing DefaultParameterSetName
function Get-Data {
    [CmdletBinding()]
    param(
        [Parameter(ParameterSetName='ByName')]
        [string]$Name,

        [Parameter(ParameterSetName='ByID')]
        [int]$ID
    )
}

# Case mismatch between DefaultParameterSetName and ParameterSetName
function Get-Data {
    [CmdletBinding(DefaultParameterSetName='ByName')]
    param(
        [Parameter(ParameterSetName='byname')]
        [string]$Name,

        [Parameter(ParameterSetName='ByID')]
        [int]$ID
    )
}

# Inconsistent casing between ParameterSetName values
function Get-Data {
    [CmdletBinding(DefaultParameterSetName='ByName')]
    param(
        [Parameter(ParameterSetName='ByName')]
        [string]$Name,

        [Parameter(ParameterSetName='byname')]
        [string]$DisplayName
    )
}

# Multiple parameter declarations in same set
function Get-Data {
    param(
        [Parameter(ParameterSetName='ByName')]
        [Parameter(ParameterSetName='ByName')]
        [string]$Name
    )
}

# Parameter set name with newline
function Get-Data {
    param(
        [Parameter(ParameterSetName="Set`nOne")]
        [string]$Name
    )
}
```

### Correct code

```powershell theme={null}
# Proper parameter set configuration
function Get-Data {
    [CmdletBinding(DefaultParameterSetName='ByName')]
    param(
        [Parameter(ParameterSetName='ByName', Mandatory)]
        [string]$Name,

        [Parameter(ParameterSetName='ByName')]
        [Parameter(ParameterSetName='ByID')]
        [string]$ComputerName,

        [Parameter(ParameterSetName='ByID', Mandatory)]
        [int]$ID
    )
}
```

## Configuration

```powershell theme={null}
Rules = @{
    PSUseConsistentParameterSetName  = @{
        Enable = $true
    }
}
```

### Parameters

* `Enable`: **bool** (Default value is `$false`)

  Enable or disable the rule during ScriptAnalyzer invocation.

## Notes

* Parameter set names are case-sensitive in PowerShell, making this different from most other
  PowerShell elements
* The first occurrence of a parameter set name in your code is treated as the canonical casing
* Parameters without `[Parameter()]` attributes are automatically part of all parameter sets
* It's a PowerShell best practice to always specify a `DefaultParameterSetName` when using parameter
  sets

## Source

This rule documentation is adapted from Microsoft's PSScriptAnalyzer documentation for
[UseConsistentParameterSetName](https://github.com/MicrosoftDocs/PowerShell-Docs-Modules/blob/main/reference/docs-conceptual/PSScriptAnalyzer/Rules/UseConsistentParameterSetName.md),
licensed under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).